Nahcolite is an ore consisting primarily of sodium bicarbonate. There are for instance vast quantities of nahcolite in the Piceance Creek Basin in Northwestern Colorado, which deposits are in the form of beds and disseminated crystals in the Saline Zone of the Green River formation.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. This deposit includes beds of trona and mixed trona and halite (rock salt or NaCl). By conservative estimates, the major trona beds contain about 75 billion metric tons of ore. A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.6NaHCO334.5H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Insolubles6.3
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water and dissolves to yield approximately 5 parts by weight sodium carbonate (Na2CO3) and 4 parts sodium bicarbonate (NaHCO3), as shown in the above analysis. The trona ore is processed to remove the insoluble material, the organic matter and other impurities to recover the valuable alkali contained in the trona.
The most valuable alkali produced from trona is sodium carbonate. Sodium carbonate is one of the largest volume alkali commodities made in the United States. In 1992, trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production. Sodium carbonate finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
A common method to produce sodium carbonate from trona ore is known as the “monohydrate process”. In that process, crushed trona ore is calcined (i.e., heated) in order to convert the bicarbonate part of the trona into crude sodium carbonate which is then dissolved in water. The resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. This process is however very energy intensive, mainly due to the calcination step, which requires the use of large quantities of coal, fuel, gas or mixtures thereof.
Attempts to reduce the energy consumption for the production of sodium carbonate have been made, by the use of electrodialytic methods. U.S. Pat. No. 4,238,305 discloses a method for recovering sodium carbonate from trona and other mixtures of sodium carbonate and sodium bicarbonate. In U.S. Pat. No. 4,238,305, sodium hydroxide is introduced into the basic compartments of an electrodialyser and soda solutions such as trona or nahcolite solutions are introduced into the acid compartments. An enriched sodium hydroxide solution is extracted from the electrodialyser. The sodium hydroxide solution is finally reacted with carbon dioxide to produce sodium carbonate. However, this process has proven to be complex to operate, in part due to the high pH in the basic compartment.